Modern complex envelope modulation schemes such as Enhanced Data rates for GSM Evolution (EDGE), Wideband Code Division Multiple Access (WCDMA), Bluetooth Enhanced Data Rate (BT-EDR), Wireless Local Area Network (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), etc. impose strict performance requirements on transceivers developed to support them, especially wireless handset transmitters. Stringent performance requirements for many aspects of polar transmitters exist as well. A circuit diagram illustrating an example prior art polar transmitter employing complex modulation based on direct phase and amplitude modulation is shown in FIG. 1. The circuit, generally referenced 10, comprises a coder 12, I and Q TX filters 14, 16, polar coordinate converter 18, local oscillator 20 and multiplier 22.
In operation, the bits bk to be transmitted are input to the coder, which functions to generate I (real) and Q (imaginary) symbols therefrom according to the targeted communications standard. The I and Q symbols are pulse-shaped and the resulting baseband signals are converted to phase (Ang{s(t)}), and magnitude (Mag{s(t)}) baseband signals by the polar coordinate converter 18. The phase data is used to control the local oscillator 20 to generate the appropriate frequency signal, which is multiplied in multiplier/mixer 22 by the magnitude data resulting in the output RF signal x(t). It is noted that this polar modulation scheme is better suited for digital implementation rather than analog implementation.
For digital polar transmitters, typical stringent performance requirements exist for modulated close-in and far-out spectra, adjacent channel power ratio (ACPR), adjacent channel leakage ratio (ACLR), error vector magnitude (EVM), phase trajectory error (PTE) and percentage power in-band. Implementation of such modern communication standards using the digital polar modulation approach is possible only if precise alignment can be maintained between the amplitude modulation (AM) and phase/frequency modulation (PM/FM) paths. This is an arduous task as both amplitude and phase (or frequency) paths comprise digital components that need to operate on coarser clock domains (i.e. clocks with time period>10 ns) for power efficiency, while complying with the stringent performance requirements of modern wireless standards. In addition, the front end circuit comprises digitally controlled analog components, such as the digitally controlled oscillator (DCO) (part of the local oscillator 20) and a digitally controlled pre-power amplifier (DPA) (part of the multiplier/mixer 22) which transforms the digital signals to the continuous-time domain with high precision.
In particular, for GSM/EDGE modulations the AM/PM alignment needs to be better than 10 nanoseconds, otherwise a degradation in the transmitter performance occurs. For WCDMA and the 4G modulations, however, the AM/PM alignment needs to be better than a nanosecond to prevent degradation in transmitter performance.
Furthermore, direct two-point modulation in a closed loop ADPLL requires modulation signals to be properly cancelled from the loop in order for the PLL to achieve optimum phase noise performance. The phase modulation accuracy requirement for modern wireless communication standards requires the direct point modulation to be at a faster rate than the reference signal typically used for reference point injection. Both these injection points need to be precisely aligned to achieve desired ADPLL operation.
The problem of time alignment described above can be generalized to any system wherein a signal is split into multiple independent paths and subsequently recombined again to reconstruct the original, but frequency translated, signal. Here to, implementation of such a system is possible only if precise alignment can be maintained between the independent signal paths. In order to produce the exact signal after all the independent paths are recombined, each and every signal path must have the exact amount of delay, otherwise the results will be distorted.
Therefore, in general, there is a need for a mechanism capable of providing precise timing alignment for a signal that has been split into multiple independent paths. In the specific case of a polar transmitter, the mechanism should be able to provide precise timing alignment for the AM and PM/FM modulation paths in a digital polar transmitter.